The present invention relates to telephone cables and, more particularly, to internally screened cables.
The history of internally screened telephone cables dates back many years and, as suggested in U.S. Pat. No. 4,340,771, can be traced from as early as 1934 up to the present. Whereas initial concern was with transmitting signals of the same frequency in two directions within the same cable, recent efforts have been directed toward providing improved carrying capacity for Pulse Code Modulation (PCM) carrier signals. In such cables the wire pairs have been divided into two groups with the pairs in one group designed for use in transmitting signals in one direction and the pairs in the other group designed to handle signals in the other direction. In Jachimowicz et al. U.S. Pat. No. 3,803,340, issued Apr. 9, 1974, there is described a cable construction that met adequately the established industry standards for 24-channel PCM carrier transmission at 772 KHz. But industry pressures for greater capacity gave rise to the invention covered in Gabriel et al. U.S. Pat. No. 4,165,442, issued Aug. 12, 1979, capable of meeting industry requirements for 48-channel PCM carrier transmission at 1.576 MHz.
Currently, the general practice is to employ separate cables to handle PCM signals on the one hand, and voice frequency (VF) signals and D.C. on the other hand. It should be understood that various control functions and the like require low frequency or D.C. signals. Consequently, there are many installations where because of the need for VF signal carrying capacity it is not economical to add an additional cable to handle the PCM carrier signals, and the advantage of PCM carrier transmission can not be obtained. Often, the underground ducts do not have the physical capacity to accommodate additional cables. Therefore, there is a considerable need for a single cable that can be substituted for the existing VF cable, that will retain the VF signal carrying capacity, and will add PCM carrier capacity.
In producing a composite cable it must be remembered that conventional PCM carrier practice calls for the installation of in-line repeaters having separate but adjacent channels within a single housing for signals in opposite directions. Thus, a weak signal from one wire pair enters the repeater, is amplified, and leaves the repeater as a strong signal while in an adjacent channel a weak signal coming in the opposite direction enters its corresponding repeater. Any crosstalk or leakage from the strong signal to the weak will cause undesirable interference. Therefore, a high degree of isolation between pairs is required. As mentioned previously, the Jachimowicz et al. and Gabriel et al. inventions provided an answer to the PCM carrier transmission problem. In addition, attempts have been made to handle both VF circuits and PCM carrier signals in a single cable by judiciously selecting the wire pairs that will carry the respective signals. However, this technique has proven to be unsatisfactory for various reasons among which is that arising from the fact that it has been difficult to control the physical location of the selected pairs along the cable throughout its entire length, and avoidance of interference was based upon maintaining a selected spacial relationship of the individual pairs with one set of pairs acting as a screen between the other pairs.
It is therefore an object of the present invention to provide a new cable construction that is capable of handling simultaneously PCM and VF signals in a convenient and economical manner with sufficiently low near end crosstalk to meet industry standards. It is an object at the same time to provide a cable that will meet other industry requirements such as that relating to protection from external hazards such as lightning or other high voltage phenomena.